Aeromonas hydrophila is a gram-negative bacterium that commonly inhabits soil and stagnant water. On extremely rare occasions, the organism can cause an aggressive, rapidly spreading, necrotizing infection in humans. The systemic signs of high fever, tachycardia, and elevation of the white blood cell count appear within 24 hours of wound contamination. If the wound is obscured by a cast, these signs erroneously may be attributed to a respiratory problem, e.g., atelectasis or fat metabolism. The organism is not usually sensitive to penicillin or cephalothin. The key to success for saving the extremity appears to be early aggressive debridement. Early diagnosis requires prompt wound inspection as soon as clinical signs of sepsis appear.